EFFECT OF MILD HYPOTHERMIA DURING CARDIOPULMONARY BYPASS ON ERYTHROCYTIC HEMOGLOBIN OXYGEN DELIVERY

Objectives: To observe and define the degree of change in hemoglobin o xygen affinity induced by hypothermic extracorporeal circulation (ECC) . Design:A prospective, nonrandomized, observational study. Setting: A single university medical center. Participants: Seventeen patients pr esenting for elective cardiac surgery. Interventions: Systemic hypothe rmia during ECC. Measurements and Main Results: During and after ECC, simultaneous arterial and mixed-venous whole-blood samples were obtain ed and immediately analyzed for gas tensions and hemoglobin saturation . Samples were obtained during the following times on ECC: initially a fter cardiopulmonary bypass onset during normothermia (37 degrees C), after cooling to 32 degrees C, and after rewarming to 37 degrees C. A fourth sample was obtained 10 to 20 minutes after discontinuation of c ardiopulmonary bypass. Extracorporeal pump flow and thermodilution-det ermined cardiac output were also recorded for calculation of oxygen de livery and consumption. Mixed-venous results were used to calculate in vivo the blood gas tension at which hemoglobin was 50% saturated with oxygen (P50). There were no differences in P50 for the 17 patients by analysis of variance (ANOVA) for repeated measures with paired t-test with Bonferroni correction. Furthermore, no change in P50 was observe d during the course of cooling and rewarming in any individual patient 's samples. Oxygen delivery decreased after hypothermia and rewarming from mild hypothermia; oxygen consumption was decreased after rewarmin g and markedly increased after discontinuation from ECC. Conclusion: M ild hypothermia to 32 degrees C during ECC does not result in in vivo alterations in oxyhemoglobin dissociation and thus does not induce cha nges in oxygen delivery to peripheral tissues. However, oxygen usage a ppears to be markedly increased after cardiopulmonary bypass. Copyrigh t (C) 1998 by W.B. Saunders Company.